Pore morphology of polar firn around closure revealed by X-ray tomography
<p>Understanding the slow densification process of polar firn into ice is essential in order to constrain the age difference between the ice matrix and entrapped gases. The progressive microstructure evolution of the firn column with depth leads to pore closure and gas entrapment. Air trans...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-07-01
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| Series: | The Cryosphere |
| Online Access: | https://www.the-cryosphere.net/12/2481/2018/tc-12-2481-2018.pdf |
| Summary: | <p>Understanding the slow densification process of polar firn into ice is
essential in order to constrain the age difference between the ice matrix and
entrapped gases. The progressive microstructure evolution of the firn column
with depth leads to pore closure and gas entrapment. Air transport models in
the firn usually include a closed porosity profile based on available data.
Pycnometry or melting–refreezing techniques have been used to obtain the
ratio of closed to total porosity and air content in closed pores,
respectively. X-ray-computed tomography is complementary to these methods, as
it enables one to obtain the full pore network in 3-D. This study takes
advantage of this nondestructive technique to discuss the morphological
evolution of pores on four different Antarctic sites. The computation of
refined geometrical parameters for the very cold polar sites Dome C and
Lock In (the two Antarctic plateau sites studied here) provides new
information that could be used in further studies. The comparison of these
two sites shows a more tortuous pore network at Lock In than at Dome C, which
should result in older gas ages in deep firn at Lock In. A comprehensive
estimation of the different errors related to X-ray tomography and to the
sample variability has been performed. The procedure described here may be
used as a guideline for further experimental characterization of firn
samples. We show that the closed-to-total porosity ratio, which is
classically used for the detection of pore closure, is strongly affected by
the sample size, the image reconstruction, and spatial heterogeneities. In
this work, we introduce an alternative parameter, the connectivity index,
which is practically independent of sample size and image acquisition
conditions, and that accurately predicts the close-off depth and density. Its
strength also lies in its simple computation, without any assumption of the
pore status (open or close). The close-off prediction is obtained for Dome C
and Lock In, without any further numerical simulations on
images (e.g., by permeability or
diffusivity calculations).</p> |
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| ISSN: | 1994-0416 1994-0424 |